Vector resonance $X_1(2900)$ and its structure

Abstract: The new vector resonance $X_1(2900)$ observed recently by LHCb in the $D<sup>{-}K<sup>{+}$ invariant mass distribution in the decay $B^{+} \to D<sup>{+}D<sup>{-}K<sup>{+}$ is studied to uncover internal structure of this state, and calculate its physical parameters. In the present paper, the resonance $X_1(2900)$ is modeled as an exotic vector state, $J^{P=1^-}$, built of the light diquark $u<sup>{T}C\gamma_5d$ and heavy antidiquark $$\overline{c} \gamma_{\mu}\gamma_5C\overline{s}<sup>{T}$$. The mass and current coupling of $X_1(2900)$ are computed using the QCD two-point sum rule approach by taking into account various vacuum condensates up to dimension $10$. The width of the resonance $X_1(2900)$ is saturated by two decay channels $X_1 \to D<sup>{-}K<sup>{+}$ and $X_1 \to \overline{D}<sup>{0}K<sup>{0}$. The strong couplings $g_1$ and $g_2$ corresponding to the vertices $X_1D<sup>{-}K<sup>{+}$ and $X_1\overline{D} <sup>{0}K<sup>{0}$ are evaluated in the context of the QCD light-cone sum rule method and technical tools of the soft-meson approximation. Results for the mass of the resonance $X_1(2900)$ $m=(2890~\pm 122)~\mathrm{MeV}$, and for its full width $\Gamma _{\mathrm{full}}=(93\pm 13)~\mathrm{MeV}$ are smaller than their experimental values reported by the LHCb collaboration. Nevertheless, by taking into account theoretical and experimental errors of investigations, interpretation of the state $X_1(2900)$ as the vector tetraquark does not contradict to the LHCb data. We also point out that analysis of the invariant mass distribution $D<sup>{+}K<sup>{+}$ in the same decay $B^{+} \to D<sup>{+}D<sup>{-}K<sup>{+}$ may reveal doubly charged four-quark structures $[uc][\overline{s}\overline{d}]$.